Exciter assemblies

ABSTRACT

An exciter assembly for supplying a field current to the rotor windings of a superconducting synchronous machine includes a pulse transformer having a stationary primary winding, a secondary winding and a tertiary winding. A switched mode power supply supplies a pulsed voltage to the primary winding of the pulse transformer. The pulsed voltage developed at the secondary winding of the pulse transformer is supplied to the rotor windings through a pair of transfer leads. A controller controls synchronous rectification of the pulsed voltage supplied to the rotor windings based on a signal from the tertiary winding of the pulse transformer.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of U.S. patent application Ser. No.11/885,085, filed Nov. 26, 2008, now U.S. Pat. No. 7,969,123 nowallowed. This application claims foreign priority PCT/GB2006/000489,filed on Feb. 13, 2006.

FIELD OF THE INVENTION

The present invention relates to exciter assemblies, and in particularto exciter assemblies for supplying current to the rotor windings of asuperconducting synchronous machine.

BACKGROUND OF THE INVENTION

It is normal for the rotor of a superconducting synchronous machine tobe located inside a cryogenic chamber (often called a cryostat) so thatthe superconducting material that is used in the rotor windings can bemaintained below its critical superconducting temperature. For a hightemperature superconducting (HTS) material such as BSCCO-2223(Bi_((2-x))Pb_(x)Sr₂Ca₂Cu₂O₁₀) or YBCO (YBa₂Cu₃O_(7-δ)) the temperaturein the cryostat can be anywhere between 27 and 110 K. The rest of thesuperconducting synchronous machine will remain at an ambienttemperature of about 300 K. For the purposes of this patentspecification, the term “cold environment” will be used to refer to thelow temperature environment inside the cryostat and the term “warmenvironment” will be used to refer to the ambient temperatureenvironment outside the cryostat.

It is essential for the operation of the superconducting synchronousmachine that the rotor windings are excited by supplying them with afield current. In a typical superconducting synchronous machine the fullfield current is supplied to the rotor windings through a pair oftransfer leads that pass from the warm environment to the coldenvironment through a wall of the cryostat. The field current isprovided by a power supply and can be supplied to the transfer leadsusing a pair of slip rings and brush contacts. The transfer leads aredesigned to minimise the stray heat transfer between the warmenvironment and the cold environment to reduce any possible adverseimpact on the performance requirements of the cryogenic cooling system.However, the transfer leads must also have a significant cross sectionalarea if they are to carry the full field current, which may be betweenten and two thousand amperes. Increasing the cross sectional area alsoincreases the amount of stray heat transfer through the transfer leads.Therefore, in practice, the design of the transfer leads must be acompromise between the need to maximise the current carrying capacitywhilst at the same time trying to minimise stray heat transfer.

U.S. Pat. No. 6,420,842 describes an exciter assembly for supplying afield current to the rotor windings of a superconducting synchronousmachine. The exciter assembly includes a transformer 106 having aprimary winding 108 and a secondary winding 112. The primary winding 108receives current from an ac power source 110 that is preferably a highfrequency excitation source (e.g., 400 Hz to 2 kHz). The transformer 106is therefore fed by a switched mode power supply. In practice, it willbe readily appreciated that the transformer 106 may or may not be apulse transformer depending on whether or not the switched mode powersupply is filtered.

An ac voltage is supplied from the secondary winding 112 to a full wavebridge rectifier 114 whose dc output is supplied to storage capacitor116. The dc voltage across the storage capacitor is not described asbeing regulated in any particular way. The dc voltage is converted to aswitched mode regulated current that flows in field winding 102 byrotating semiconducting power devices 120, 122 and 138 which can eitherbe located in a cryogenic environment or a warm environment as required.When the rotating semiconductor power devices 120, 122 and 138 arelocated in a cryogenic environment, a pulsed current flows in thetransfer leads that pass between warm and cryogenic environments. Whenthe rotating semiconductor power devices 120, 122 and 138 are located ina warm environment, a substantially non-pulsing current flows in thetransfer leads that pass between warm and cryogenic environments. Inboth cases, the switched mode regulator power semiconductor devices arein a rotating environment and carry field current. A field currentregulation process using a telemetry link comprising stationary machinecontroller interface 134 and a rotating field coil controller interface130 that employ pulse code modulated carrier infrared opticaltransmission and reception in order to bi-directionally transferregulator signals between stationary and rotating environments. Acurrent sensor 132 is located in the rotating environment and it isnecessary to transfer data from this sensor via the telemetry link toenable closed loop regulation of field current to be performed.

Accordingly, there is a need for an alternative exciter assembly thatdoes not require switched mode regulator power semiconductor devices anda field current transducer to be in a rotating environment and for theseto have to communicate with the stationary environment in order toenable closed loop regulation of field current to be performed.

SUMMARY OF THE INVENTION

The present invention provides an exciter assembly for supplying a fieldcurrent to the rotor windings of a superconducting synchronous machine,the exciter assembly comprising:

-   -   a pulse transformer having a primary winding and a secondary        winding;    -   a switched mode power supply for supplying a pulsed dc current        to the primary winding of the pulse transformer; and    -   a pair of transfer leads for supplying a pulsed dc current from        the secondary winding of the pulse transformer to the rotor        windings.

The rotor windings will be located in a cryogenic region of thesuperconducting synchronous machine, such as inside a cryogenic chamberor cryostat. The cryogenic region will be referred to below as the “coldenvironment”.

The primary winding of the pulse transformer is preferably stationaryand the secondary winding of the pulse transformer preferably rotates inuse. Both the primary and secondary windings of the pulse transformer,as well as the switched mode power supply, are preferably located in the“warm environment” outside the cryogenic region. The transfer leads areused to transfer the pulsed dc current between the warm environment andthe cold environment, usually through a wall of the cryogenic chamber orcryostat.

The field current is preferably provided by a switched mode power supplyhaving a significant forcing voltage so that, at start up, a workingfield current can be established in the rotor windings in a relativelyshort period of time. During normal operation of the superconductingsynchronous machine, the transfer leads will only carry the fieldcurrent for a small proportion of the time.

The exciter assembly preferably further comprises a rectifiersemiconductor device in series with the rotor winding and a flywheelsemiconductor device in parallel with the rotor winding. The rectifiersemiconductor device can be a thyristor, a Gate Turn Off Thyristor (GTO)or other device with similar reverse blocking and gate turn oncharacteristics. The flywheel semiconductor device can be a JunctionField Effect Transistor (JFET) or Vertical Junction Field EffectTransistor (VJFET), for example. The JFET and VJFET devices do notsuffer from the presence of the parasitic body diode that is an inherentfeature of Metal Oxide Silicon Field Effect Transistors (MOSFETs) andthe silicon carbide derivatives thereof. Because they do not have aparasitic body diode, JFET and VJFET devices are able to block voltagesof both polarities under gate control. Moreover, JFET and VJFET devicesdo not suffer the reverse recovery performance limitations that areimposed by parasitic body diodes of other devices.

The rectifier semiconductor device and the flywheel semiconductor deviceare preferably controlled for synchronous rectification of the pulsedvoltage supplied to the rotor windings. In contrast to the exciterassembly disclosed in U.S. Pat. No. 6,420,842, the synchronousrectification is controlled by an electronic controller, which uses atiming signal taken from a tertiary winding of the pulse transformer andsupplies gate pulses to the rectifier semiconductor device and theflywheel semiconductor device to turn them on and off at appropriatetimes.

The exciter assembly preferably further comprises a snubber including aninductor in series with the rotor windings and a capacitor in parallelwith the rotor windings.

To protect the rotor windings from damage in the event that incipientquench occurs (i.e. the superconducting material forming the rotorwindings starts to become locally resistive either because thetemperature rises above the critical temperature or the current densityrises above the critical current density or because of a winding defect,for example) the switched mode power supply can be switched off and aswitchable means including a field discharge resistor (sometimes calleda dump resistor) can be employed in series with the rotor windings inorder to force the field current down. The dump resistor is preferablyin series with a semiconductor switch device such as a thyristor, GateTurn Off Thyristor (GTO) or other device with similar reverse blockingand gate turn on characteristics, for example. Synchronous rectificationshould also be inhibited if incipient quench is detected by turning offboth the rectifier and flywheel semiconductor devices. Mechanical meansfor the passive protection of the rotor windings may also be provided.For example, metallic buffer layers may be deposited over HighTemperature Superconductor (HTS) films in order to provide aconventional electrically and thermally conductive material that isintimate contact with the HTS films. It is essential that the respectiveresponses of these passive protection means, the incipient quenchdetection system and the dump resistor switching device are co-ordinatedin order to provide effective protection against incipient quench.

The controller preferably controls the operation of the semiconductorswitch device, and optionally the rectifier and flywheel semiconductordevices, based on a current feedback signal indicative of the fieldcurrent in the rotor windings during a first period of time when a pulseof voltage is supplied to the rotor windings and a voltage feedbacksignal indicative of the voltage across the rotor windings during asecond period of time when a pulse of voltage is not being supplied tothe rotor windings.

The current feedback signal can be derived from a current transducer inseries with the primary winding of the pulse transformer and the voltagefeedback signal can be derived from a voltage transducer in parallelwith the rotor windings.

The rotor windings are preferably formed from an HTS material such asBSCCO or YBCO, for example. Other possible HTS materials include membersof the rare-earth-copper-oxide family. It will be readily appreciatedthat the superconducting field windings can also be formed from a LowTemperature Superconducting (LTS) material such as Nb₃Sn and NbTi or aMedium Temperature Superconducting (MTS) material such as MgB₂(magnesium diboride).

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will now be described, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram showing the topology of an exciterassembly according to the present invention in outline;

FIG. 2 is a schematic diagram showing the topology of the exciterassembly of FIG. 1 in detail;

FIG. 3 is a diagram showing the pulse width modulated waveforms of thecurrent flowing in the diode rectifier, flywheel diode and fieldwinding;

FIG. 4 is a schematic diagram showing the controller that is usedcontrol the operation of the exciter assembly of FIG. 1;

FIG. 5 is a diagram comparing the field winding voltage and currenttraces for two different field discharge resistors and a device having anon-linear resistance;

FIG. 6A is a perspective view of a first pulse transformer;

FIG. 6B is a cross section of the pulse transformer of FIG. 6A;

FIG. 7A is a perspective view of a second pulse transformer;

FIG. 7B is a cross section of the pulse transformer of FIG. 7A;

FIG. 8A is a perspective view of a third pulse transformer;

FIG. 8B is a cross section of the pulse transformer of FIG. 8A;

FIG. 9A is a perspective view of a fourth pulse transformer; and

FIG. 9B is a cross section of the pulse transformer of FIG. 9A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The basic topology and operation of an exciter assembly according to thepresent invention will now be explained with reference to FIG. 1. Theexciter assembly includes a switched mode power supply 100 that islocated at ambient temperature on, or near to, the stator of asuperconducting synchronous machine (the “machine”). The exciterassembly also includes a pulse transformer 101 and a first powerassembly 103 that are located at ambient temperature.

The output of the switched mode power supply 100 is connected to theprimary winding of the pulse transformer 101. The secondary winding ofthe pulse transformer 101 is connected to the first power assembly 103.The primary winding remains stationary while the secondary winding andthe power assembly 103 are secured to the rotor 102 of the machine androtate with it. Field current is supplied from the first power assembly103 to a second power assembly 104 that is located inside the cryostat17 using a pair of transfer leads 14 which pass through a wall of thecryostat.

The second power assembly 104 controls the circulation of the fieldcurrent (commonly known as flywheel action) in the rotor windings of themachine, represented in FIG. 1 by a field winding 16, when the firstpower assembly 103 is not supplying current to the second power assemblyand the field winding. The rotor windings are formed from a HighTemperature Superconducting (HTS) material such as BSCCO-2223 or YBCOwires and tapes. BSCCO wire is made from (Bi, Pb)₂Sr₂Ca₂Cu₃O₁₀ filamentsin a metal matrix and has a critical temperature of 110 K but typicallyoperates in motors and generators at less than 40 K. YBCO wires or tapescould operate at higher temperatures in a motor or generator. YBCO asthin film could be directly deposited on a machine rotor. One possiblesupplier of BSCCO-2223 or YBCO wire is American Superconductor (AMSC),HTS Wire Manufacturing Facility of Jackson Technology Park, 64 JacksonRoad, Devens, Mass. 01434-4020, United States of America.

The switched mode power supply 100 supplies a pulsed voltage to theprimary winding of the pulse transformer 101. The pulsed voltage istransferred between the stationary parts of the exciter assembly (i.e.the switched mode power supply 100 and the primary winding of the pulsetransformer 101) to the rotating parts of the exciter assembly (i.e. thesecondary winding of the pulse transformer 101 and the first and secondpower assemblies 103 and 104) across the air gap of the pulsetransformer 101. The pulsed voltage is then transferred between theparts of the exciter assembly that are located in the warm environment(i.e. the switched mode power supply 100, the pulse transformer 101 andthe first power assembly 103) and the parts of the exciter assembly thatare located in the cold environment inside the cryostat 17 (i.e. thesecond power assembly 104) through the transfer leads 14. The fieldcurrent supplied to the field winding 16 is regulated by stationarysemiconductor devices within the switched mode power supply 100.Synchronous rectification of the field current is performed by deviceswithin the first and second power assemblies 103 and 104. The firstpower assembly 103 does not normally regulate the field current butinitiates commutation of the synchronous rectification of the fieldcurrent if incipient quench of the field winding 16 is detected. Themethod by which incipient quench of the field winding 16 is detectedwill be described in more detail below.

The carrier frequency of the pulse width modulation is sufficiently highto minimise the size and dissipation of the pulse transformer, and alsoto permit structural attenuation to minimise the generation ofstructure-borne noise and vibration. If the pulse transformer issupplied at say 60 Hz, structural vibration in the pulse transformerwill be excited at a series of harmonic frequencies having a fundamentalfrequency of 60 Hz, for example 60, 120, 180, 240 Hz and so on.Furthermore, the resultant ripple in the field current will also becomposed of the same harmonic series and these will generate forces thatwill excite structural vibration in the machine. It is well known thatstructural responses to applied forces are strongly frequency dependantand complex, but are generally characterised by having resonance bandsat particular frequencies where structural displacements are amplified.At frequencies well below the resonance bands, structural displacementsstay constant as frequency is increased. At frequencies above theresonance bands, structural displacements are more significantly reducedas frequency is increased. As the excitation frequency is increased, themechanisms of mass, compliance and damping cause beneficial structuralattenuation. It is therefore beneficial to employ as high an excitationfrequency as is practical. Moreover, it is beneficial for the excitationfrequency to be adjustable so as to avoid coincidence of the excitationand structural resonant frequencies. A typical value for the carrierfrequency of the pulse width modulation would be 8 kHz.

The construction and operation of the exciter assembly will now bedescribed in more detail with reference to FIG. 2. A switched mode powersupply of the industry-standard “forward converter” (or Buck converter)type includes a dc voltage supply 1, a dc supply capacitor 2, a corereset resistor 3, a core reset capacitor 4, a switching transistor 5 anda core reset diode 6. The core reset components 3, 4 and 6 are only atypical industry-standard implementation and their actual designtopology will be greatly dependent upon equipment rating, particularlythe modulation range of the pulse width modulation employed by theswitched mode power supply. In particular, when modulation depthincreases, the reset voltage must also be increased so that the corereset volt-second integral is equal and opposite to the core setvolt-second integral. The requirement to reset the core after each pulseof “forward conversion” imposes a practical restriction on themodulation depth and those conversant with switched mode magneticcircuit design will be aware of appropriate core reset techniques.

The switched mode power supply feeds a pulse width modulated regulatedvoltage to the pulse transformer having a primary winding 7, a primarymagnetic core 8, a secondary magnetic core 9 and a secondary winding 10.The primary winding and magnetic core 7 and 8 are stationary while thesecondary magnetic core and winding 9 and 10 rotate with the rotor ofthe machine. The primary and secondary magnetic cores 8 and 9 areseparated by a small air gap. It will be readily appreciated that thereis no requirement to provide an air gap between the magnetic cores 8 and9, but even when the pole faces of the magnetic cores are in slidingcontact with each other, pole face asperities prevent an intimatecontact between them and a thin interface region with relatively lowaverage magnetic permeability is formed having air gap-likecharacteristics. Since variations in the air gap characteristics wouldhave an adverse effect on the performance of the pulse transformer, thealternative implementations described below maintain a controlledsliding interface between the primary and secondary magnetic cores.

The output of the secondary winding 10 of the pulse transformer is alsoa pulse width modulated voltage and is related to the voltage at theprimary winding 7 according to the ratio of turns on the primary andsecondary windings of the pulse transformer. Similarly, any current inthe secondary winding 10 will be mirrored in the primary winding 7according to the ratio of turns, and taking into account the fact thatthe primary winding will contain a magnetising current component havinga predictable characteristic. The pulse width modulated voltage has twostates, commonly referred to as the “mark” state and the “space” state,respectively. The “mark” state exists when the switching transistor 5 ofthe switched mode power supply is in its “on” state and the power supplyvoltage at the power supply capacitor 2 is applied to the primarywinding 7 of the pulse transformer. The “space” state exists when theswitching transistor is in its “off” state and the core reset componentsdefine the voltage that is applied to the primary winding 7 of the pulsetransformer. The core of the pulse transformer is often stated as being“set” during the “mark” state and “reset” during the “space” state. Itshould be noted that the current pulses in the primary and secondarywindings 7 and 10 of the pulse transformer are unidirectional, whereasthe voltage reverses at “mark” state and “space” state transitions, asdescribed above.

During start up of the machine it is necessary to bring the fieldcurrent in the field winding 16 up to the required level. In idealcircumstances this might be achieved by applying a single voltage pulselasting several minutes to “ramp up” the field current. However, inpractice the application of a single voltage pulse for this length oftime would quickly lead to saturation in the pulse transformer and so aseries of shorter pulses must be applied over the same or a slightlylonger period of time to ramp up the field current in a series of steps.Further practical limitations that may apply to the duration of thefield current “ramp up” are:

-   -   (i) the field winding insulation capability;    -   (ii) the peak current rating of the switched mode power supply;    -   (iii) the short duration overload current rating of the pulse        transformer; and    -   (iv) the thermal loading of the transfer leads 14.

Limitation (i) is not normally an overriding design issue because thefield winding insulation must infrequently withstand the forcing voltageassociated with incipient quench protection. Insulation life expectancyis non-linearly related to applied voltage and frequency, but is not anissue providing the period of time over which the “ramp up” takes placeis many times longer than the “field discharge” time. Limitations (ii)and (iii) are simply issues of space availability and cost according toconventional design guidelines. Limitation (iv) is described in moredetail below.

The field current in the field winding 16 will only dissipate veryslowly when no voltage is applied. Therefore, once the field current hasreached the acceptable level it is sufficient to apply a series of shortvoltage pulses at relatively long intervals (perhaps in the order ofseveral minutes or hours) to keep the field current at the substantiallythe same level. This process is often referred to as “pumping” the fieldcurrent. The exciter assembly must therefore be able to operate in twodifferent modes, namely a “start up” mode where the field current isramped up to an acceptable level and a “pumping” mode when the machineis operating normally and the level of the field current is maintainedby supplying a number of short voltage pulses. The exciter assembly mayalso operate in a third mode (a “protection” or “dump” mode) whenincipient quench is detected.

The different operating modes of the exciter assembly will now beexplained with reference to the pulse width modulated voltage suppliedto the field winding 16. The period of time during which a voltage pulseis applied to the field winding 16 is the “mark” state and the period oftime between voltage pulses is the “space” state. The first powerassembly 103 includes a rectifier semiconductor device 11 and the secondpower assembly 104 includes a flywheel semiconductor device 15. Therectifier semiconductor device 11 and flywheel semiconductor device 15are used in the synchronous rectification of the field current and havea general step-down (or “Buck”) converter topology. The inductor 12 andcapacitor 13 are used to eliminate the magnitude of higher frequencycomponents of the pulse width modulated voltage supplied to the leads 14by forming a low pass filter and thereby also acting as a snubber(switching aid network) for the rectifier semiconductor device 11 andthe flywheel semiconductor device 15. This in turn limits the magnitudeof higher frequency components of field current flowing in the fieldwinding 16. During a “mark” state when the switching transistor 5 of theswitched mode power supply is in the “on” state, the voltage pulsedeveloped at the secondary winding 10 passes through the rectifiersemiconductor device 11 and through the transfer leads 14 to the fieldwinding 16. During a “space” state when the switching transistor 5 ofthe switched mode power supply is in the “off” state, the field currentin the field winding 16 flows in a closed path through the flywheelsemiconductor device 15. FIG. 3 shows how the field current (labelled “Ifield (16)”) flows through the rectifier semiconductor device 11 duringthe “mark” state and then through the flywheel semiconductor device 15during the “space” state. The bottom waveform shows how the fieldcurrent in the field windings only dissipates very slowly during the“space” state when no voltage is applied.

The waveforms shown in FIG. 3 are for the exciter assembly operating inthe “pumping” mode. When the exciter assembly is operating in the “startup” mode then it will be readily appreciated that the period of timebetween voltage pulses (or in other words the “space” state) will begreatly reduced so that the field current can be ramped up to therequired level in a series of steps. The ratio of the period of timeduring which a voltage pulse is applied to the period of time betweenvoltage pulses (i.e. the ratio of the “mark” state to “space” state)will therefore be different depending on whether the exciter assembly isoperating in the “start up” mode or the “pumping” mode.

Instead of having to cope with the full field current, the transferleads 14 only have to supply short pulses of voltage to the fieldwinding 16 at fairly infrequent intervals when the exciter assembly isoperating in the “pumping” mode. This leads to a reduction in thecontinuous rms stray heat transfer through the transfer leads 14. Thelimitation (iv) mentioned above is not severe because the transfer leads14 are dimensioned to limit the continuous rms heat losses in order notto be dominant in cryo-plant rating and size. The thermal time constantof the bulk of the field winding 16 is many times the “ramp up” durationand transfer lead 14 current overloads do not have an immediate criticaleffect on the temperature of the field winding. However, care must betaken to avoid excessive heat input into the connection between thetransfer leads 14 and the ends of the field winding 16. The risk oflocal overheating can be avoided by appropriate design of the cryogenicfluid cooling circuit that cools the interior of the cryostat. In anyevent, the transfer leads 14 must be able to withstand the fielddischarge current associated with incipient quench protection, and it issignificant that the requirement for quench protection may be as aresult of the failure of the cryogenic fluid cooling circuit.

The control of the exciter assembly will now be described in more detailwith reference to FIG. 4. The pulse transformer includes a tertiarywinding 34 and the output from the tertiary winding is also a pulsewidth modulated voltage that is related to the voltage at the primarywinding 7. A controller 35 uses the voltage at the tertiary winding as apower supply and a synchronisation reference.

As described above, the rectifier semiconductor device 11 and flywheelsemiconductor device 15 are mainly used as synchronous rectifiers andhave their gate pulses synchronised to the voltage output of thetertiary winding 34. The gate terminals of the rectifier semiconductordevice 11 and the flywheel semiconductor device 15 also allow thecontroller 35 to initiate forced commutation when required.

The controller 35 synchronously samples a current feedback signal 37that is derived from a current transducer 36 during the “mark” state.During the “mark” state the current in the transducer 36 is equivalentto the field current flowing in the field winding 16 and the controller35 is therefore able to sense the field current throughout the “mark”state. The controller 35 also synchronously samples a voltage feedbacksignal 39 derived from a voltage transducer 38 during the “mark” stateand the “space” state. The voltage feedback signal 39 is transferredbetween the cold environment and the warm environment by a lead 40. Bylocating the voltage transducer 38 next to the field winding 16, errorsresulting from stray voltage drops outside the field winding 16 may beavoided, but it must be noted that care must be taken to avoid errorsarising from thermocouple effects at voltage sensing connections.Alternatively, it is also possible to locate the voltage transducer 38outside the cryostat as long as leakage current into the rectifiersemiconductor device 11 and the semiconductor switch device 19 aresufficiently low not to cause excessive voltage drop in the transferleads 14, and providing the controller 35 only samples the voltagefeedback signal 39 during the “space” state. Locating the voltagetransducer 38 outside the cryostat removes the need for the lead 40 witha corresponding simplification of the exciter assembly design.

The controller 35 uses the current feedback signal 37 and the voltagefeedback signal 39 in combination with a suitable computationalalgorithm to estimate the resistance and the inductance of the fieldwinding 16. These estimates are updated at the pulse width modulationcarrier frequency of the switched mode power supply. The controller 35compares the estimates with pre-determined values for the resistance andinductance in order to detect incipient quench of the HTS material inthe field winding 16. If incipient quench is detected then thesynchronous rectification of the rectifier semiconductor device 11 andthe flywheel semiconductor device 15 is inhibited by turning off both ofthe devices by gate control.

When synchronous rectification is inhibited, the switched mode powersupply will supply only magnetising current to the primary winding 7 ofthe pulse transformer. There is no requirement to cease operation of theswitched mode power supply following inhibition of synchronousrectification because the rectifier superconductor device 11 canwithstand the open circuit output voltage of the switched mode powersupply as coupled by the pulse transformer. However, power supplyshutdown can simply be implemented within the switched mode power supplyif it is considered advantageous to do so. Moreover, there is norequirement for rotor telemetry to be provided in order to advise theoperator or “trip” the switched mode power supply following thecommencement of incipient quench protection, because the switched modepower supply is able to determine that synchronous rectification hasbeen inhibited by sensing its load impedance. It is commonplace forswitched mode power supply equipment to sense its output current andsupply voltage and use these to determine the load impedance from aknowledge of modulation depth and an estimate of the magnetising currentin the primary winding 7 of the pulse transformer. This allows theswitched mode power supply to detect extremes of load impedance such asopen circuit, for example.

When the rectifier semiconductor device 11 and the flywheelsemiconductor device 15 are simultaneously turned off, the controller 35can apply a gate pulse 42 to a switching device 19 whilst the voltageacross the field winding 16 rises rapidly with a polarity that enablesthe switching device to conduct. The conduction of the switching device19 causes the field discharge resistor 18 (sometimes called a dumpresistor) to be connected in series with the field winding 16. Thevoltage drop across the resistor 18 causes the field current in thefield winding 16 to decay approximately exponentially with respect totime. The resistance is determined according to a compromise between theinsulation design and the fault dissipation constraints of the fieldwinding 16. A small value of resistance causes the peak winding voltageto be restricted while the field current discharge time constant isincreased. On the other hand, a large value of resistance causes thepeak winding voltage to increase while the field current discharge timeconstant is reduced. The peak winding voltage influences the insulationdesign and the field current discharge time influences the faultdissipation in the HTS material. The relationship between these twoinfluences can be beneficially altered by employing a non-linearresistance in place of the conventional field discharge resistor 18. Agroup of parallel-connected Metal Oxide Varistors (MOVs) or othernon-linear surge arrester devices, with similar positive temperaturecoefficient of avalanche voltage and positive slope resistance, may beused to provide a substantially constant winding voltage during thefield current discharge period. This has the effect of reducing theratio of the fault dissipation in the HTS material with respect to thepeak insulation voltage.

With reference to FIG. 5, when a linear resistor device (such as thefield discharge resistor 18 mentioned above) is used for field dischargepurposes then the field winding voltage and the current both decayexponentially with time. However, if a non-linear resistor device isused then the field winding voltage has a rectangular decay and thecurrent decays according to a ramp with time. FIG. 5 shows three fieldwinding voltage and current traces for three different resistor devices.The first trace (labelled “Exponential decay with shorter timeconstant”) depicts the field winding voltage as it decays from aninitial voltage V1 to zero in a time T2, as would be the case when ahigher value field discharge resistor is employed. The second trace(labelled “Exponential decay with longer time constant”) depicts thefield winding voltage as it decays from an initial voltage V2 to zero ina time T1, as would be the case when a lower value field dischargeresistor is employed. The third trace (shown in dashed lines andlabelled “Rectangular decay”) depicts the field winding voltage as itdecays from an initial voltage V3 to zero at a time T3, as would be thecase when a non-linear device is employed. Note that in the second tracethe insulation stress has been reduced relative to the first tracebecause the peak voltage has been reduced from V1 to V2. However, theduration of the current decay shown below the voltage traces has beenincreased from T2 to T1, thus placing the field winding under greaterthermal stress with a consequent risk of incipient quench. Note alsothat in the third trace, the peak voltage V3 is lower than both V1 andV2 and the duration of the current decay T3 is also the lower than bothT1 and T2. The use of a non-linear resistor device in place of the fielddischarge resistor 18 therefore has the benefit of simultaneouslyreducing both the insulation stress and the thermal stress on the fieldwinding.

The flywheel semiconductor device 15 can be a Junction Field EffectTransistor (JFET) or a Vertical Junction Field Effect Transistor(VJFET), which may be implemented as a depletion mode device or anenhancement mode device with synchronous gating being provided by thecontroller 35. In fact, a number of cryogenic switch implementations arepossible and it is well known that majority carrier semiconductordevices have a positive thermal coefficient of “on” state voltage dropat temperatures above carrier freeze out, which occurs at approximately50 degrees K in silicon devices. This positive thermal coefficientcauses self-stabilisation and uniformity of “on” state current densityover the whole die area. Moreover, the same effect causesparallel-connected groups of die to share current equally, providingcooling arrangements, interconnection geometry and gate drivearrangements are carefully specified in order to achieve thermal,mechanical and electrical symmetry. Such semiconductor devices also havea very high switching speed. The use of a JFET, VJFET or Metal OxideSemiconductor Field Effect Transistor (MOSFET) under cryogenicconditions therefore facilitates the fabrication of large die areaswitches with very low dissipation, high current rating and highswitching speed.

The rectifier semiconductor device 11 and switching device 19 can be athyristor, Gate Turn Off Thyristor (GTO) or any other suitablesemiconductor device with similar reverse blocking and gate turn oncharacteristics. Except when synchronous rectification of the rectifiersemiconductor device 11 and the flywheel semiconductor device 15 must beinhibited by turning off both devices by gate control, the phaserelationship of the gate pulse 43 applied to the flywheel semiconductordevice 15 by the controller 35 is synchronised to the operation of thesecondary winding 10 of the pulse transformer and the rectifiersemiconductor device 11 because of the precise phasing of the tertiarywinding 34 of the pulse transformer with respect to the secondarywinding. Similarly, the phase relationship of the gate pulse 41 appliedto the rectifier semiconductor device 11 by the controller 35 issynchronised to the operation of the secondary winding 10 of the pulsetransformer and the flywheel semiconductor device 15 because of theprecise phasing of the tertiary winding 34 of the pulse transformer withrespect to the secondary winding 10.

When the flywheel semiconductor device 15 is a depletion mode device itwill revert to its naturally “on” state except when the gate pulse 43 isapplied to switch it to its “off” state. Gate pulses 41 and 43 areapplied simultaneously and by this means simultaneous conduction ofsemiconductor devices 11 and 15 is avoided. When the flywheelsemiconductor device 15 is an enhancement mode device it will revert toits naturally “off” state except when the gate pulse 43 is applied toswitch it to its “on” state. Gate pulses 41 and 43 are applied inanti-phase and by this means simultaneous conduction of semiconductordevices 11 and 15 is avoided.

The gate pulse 43 is transferred between the cold environment and thewarm environment by a lead 44.

The active incipient quench protection described above is particularlyadvantageous because the detection can be performed very rapidly at thepulse width modulation carrier frequency. It will be readily appreciatedthat the HTS material in the rotor windings may also be provided withpassive protection, perhaps in the form of a buffer layer of copper forexample.

The pulse transformer may be implemented in a number of different waysand some of the alternatives will now be described. In all cases, theprimary system (i.e. the primary winding and its associated magneticcore) is stationary and is located in the warm environment and thesecondary system (i.e. the secondary winding and its associated magneticcore) rotates with the rotor of the machine and is located in the warmenvironment.

A variety of different pulse transformer will now be described withreference to FIGS. 6 to 9. All of the magnetic circuits below are formedfrom ferrite material.

Referring first to FIGS. 6A and 6B, a first pulse transformer includes aprimary core 18 which is a U-section solid of revolution to provide acavity 18 b into which a solenoidal primary winding 19 is inserted. Theprimary connections 19 a are accessible via a recess or aperture 18 a inthe primary core 18. The first pulse transformer also includes asecondary core 21 which is a U-section solid of revolution to provide acavity 21 b into which a solenoidal secondary winding 20 is inserted.The secondary connections 20 a are accessible via a recess or aperture21 a in the secondary core 21.

Referring now to FIGS. 7A and 7B, a second pulse transformer includes anindustry-standard U-shaped primary core 22. A solenoidal primary winding22 a is formed around the core 22. Two concentrically disposed primarypole piece rings 23 and 24 are formed from similar ferrite material andare connected to the core 22 in order to distribute the flux from thepole faces of the core. A single core 22 is shown but in practiceseveral cores may be connected to the pole piece rings 23 and 24 inorder to reduce the circumferential component of flux in the pole piecerings. Primary connections 22 b are brought out of the winding 22 a. Thesecond pulse transformer also includes an industry-standard U-shapedsecondary core 27. A solenoidal secondary winding 27 a is formed aroundthe core 27. Two concentrically disposed secondary pole piece rings 25and 26 are formed from similar ferrite material and are connected to thecore 27 in order to distribute the flux from the pole faces of the core.A single core 27 is shown but in practice several cores may be connectedto the pole piece rings 25 and 26 in order to reduce the circumferentialcomponent of flux in the pole piece rings. Secondary connections 27 bare brought out of the winding 27 a. When multiple primary and secondarycores 22 and 27 are employed then they are regularly spaced in order toreduce the circumferential component of flux in the primary and secondpole piece rings. The axial and radial positions of the primary andsecondary components of the second pulse transformer are regulated inorder to maintain acceptable air gap, friction and wear characteristics.The second pulse transformer shown in FIGS. 7A and 7B is fullysymmetrical but it is also possible that unequal numbers of primary andsecondary cores can be used in order to reduce the effects of cyclicvariation in the flux linkage when the rotating cores are in motion.

The third pulse transformer shown in FIGS. 8A and 8B is an asymmetricalvariant of the first pulse transformer where the primary core 18 isreplaced by ring core 29 of rectangular radial cross section. In thiscase, the solenoidal primary winding 28 is housed within the cavity 21 bof the secondary core 21, adjacent to the secondary winding 20. Theaxial positions of the primary and secondary windings 28 and 20 areregulated in order to avoid friction and wear because they are in closeproximity to one another.

Referring now to FIGS. 9A and 9B, a fourth pulse transformer has anouter core which is a U-section solid of revolution formed in two matingsections 33 a and 33 b. The core provides a cavity 33 c into whichconcentrically disposed primary and secondary solenoidal windings 31 and32 are inserted. The magnetic circuit is completed by an inner ring core30. The primary connections 32 a are accessible via a recess of aperture33 d in the core section 33 a. The secondary connections 31 a areaccessible via a recess or aperture 30 a in the inner ring core 30. Theaxial and radial positions of the primary and secondary cores areregulated in order to maintain acceptable air gap, friction and wearcharacteristics. The axial and radial positions of the primary andsecondary windings 31 and 32 are also regulated to avoid friction andwear because they are in close proximity to one another and to the wallsof the cavity 33 c.

1. A superconducting synchronous machine having rotor windings formedfrom a superconducting material, and an exciter assembly for supplying afield current to the superconducting rotor windings, the exciterassembly comprising: a pulse transformer having a primary winding, asecondary winding and a tertiary winding; a switched mode power supplyfor supplying a primary pulsed ac voltage to the primary winding of thepulse transformer; a rectifier for deriving a pulsed dc voltage bysynchronous rectification of a secondary pulsed ac voltage supplied bythe secondary winding of the pulse transformer; a pair of transfer leadsfor supplying the pulsed dc voltage derived by the rectifier to thesuperconducting rotor windings; and a controller for controllingsynchronous rectification of the secondary pulsed ac voltage by therectifier based on a signal from the tertiary winding of the pulsetransformer.
 2. The superconducting synchronous machine of claim 1,wherein the superconducting rotor windings are located in a cryogenicregion.
 3. The superconducting synchronous machine of claim 2, whereinthe primary winding and the secondary winding of the pulse transformerare located outside the cryogenic region.
 4. The superconductingsynchronous machine of claim 2, wherein the switched mode power supplyis located outside the cryogenic region.
 5. The superconductingsynchronous machine of claim 2, wherein the rectifier comprises arectifier semiconductor device in series with the superconducting rotorwindings and a flywheel semiconductor device in parallel with thesuperconducting rotor windings.
 6. The superconducting synchronousmachine of claim 5, wherein the rectifier semiconductor device islocated outside the cryogenic region.
 7. The superconducting synchronousmachine of claim 5, wherein the flywheel semiconductor device is locatedinside the cryogenic region.
 8. The superconducting synchronous machineof claim 1, wherein the superconducting rotor windings are formed from ahigh temperature superconducting (HTS) material.
 9. The superconductingsynchronous machine of claim 1, wherein the superconducting rotorwindings include means for protecting the superconducting rotor windingsagainst damage in the event of incipient quench.